Paweas
I wasn't saying that fuel burn was unimportant, or that we would deliberately to operate inefficiently by that statement.
I will try again .... the type is irrevelent to the decision. You take the fuel flow and groundspeed for the type/any type at the different levels and do the maths. If it makes sense to fly lower and there are no other considerations, then go for it. The fact that a 747 may burn 10 tonnes/hour and a 330 only 4 and an empty 737 2 tonnes is neither here nor there, it is the efficiency for whatever you are flying that counts.
Most airlines in this day and age have very sophisticated flight planning systems in place in order to minimise total costs. As well as considering winds and levels they will consider flight time for maintenance (and crew overtime costs), for schedule regularity, for ATC overflight charges, air temperatures (think BA777 Jan08!) and other things. As a B747 Captain, I endeavour likewise to deliver the passengers on time having had a comfortable and enjoyable flight having been well looked after adn to have used minimum fuel - ie at least cost to my employer (and the worlds oil reserves).